The demand for progressively smaller, less expensive, and more powerful electronic products creates the need for smaller geometry integrated circuits (ICs), and larger substrates. It also creates a demand for a denser packaging of circuits onto IC substrates. The desire for smaller geometry IC circuits requires that the dimension of interconnections between the components and the dielectric layers be as small as possible. Therefore, recent research continues to focus on the use of low resistance materials (e.g., copper) in conjunction of with insulating materials with low dielectric constant (k) between the metal lines.
The use of low resistance materials is needed because of the reduction of the cross section area of via interconnects and connecting lines. The conductivity of interconnects is reduced as the surface area of interconnects is reduced, and the resulting increase in interconnect resistivity has become an obstacle in IC design. Conductors having high resistivity create conduction paths with high impedance and large propagation delays. These problems result in unreliable signal timing, unreliable voltage levels, and lengthy signal delays between components in the IC. Propagation discontinuities also result from intersecting conduction surfaces that are poorly connected, or from the joining of conductors having highly different resistivity characteristics.
There is a need for interconnects and vias to have low resistivity, and the ability to withstand volatile process environments. Aluminum and tungsten metals are often used in the production of integrated circuits for making interconnections or vias between electrically active areas. These metals have been used for a long time in the production environment because the processing technologies for these metals were available. Experience and expertise on these metals have also been acquired in the process due to the long-term usage.
Copper is a natural choice to replace aluminum in the effort to reduce the size of lines and vias in an electrical circuit. The conductivity of copper is approximately twice that of aluminum and over three times that of tungsten. As a result, the same current can be carried through a copper line having half the width of an aluminum line.
However, there have been problems associated with the use of copper in IC processing. Copper poisons the active area of silicon devices, creating unpredictable responses. Copper also diffuses easily through many materials used in IC processes and, therefore, care must be taken to keep copper from migrating.
Various means have been suggested to deal with the problem of copper diffusion into integrated circuit materials. Several materials, including metals and metal alloys, have been suggested for use as barriers to prevent the copper diffusion process. The typical conductive diffusion barrier materials are TiN, TaN and WN. Addition of silicon into these materials, TiSiN, TaSiN, WSiN, could offer improvement in the diffusion barrier property. For non-conductive diffusion barrier, silicon nitride has been the best material so far.
Diffusion barrier materials could be deposited by the chemical vapor deposition technique. For example, in the case of TiN CVD deposition, a precursor that contains Ti and optionally nitrogen, is used. The precursor decomposes to form a TiN layer on selected surfaces. Precursor by-products (products formed as the precursor decomposes that do not participate in the reactions) and reaction by-products (products formed from the reaction that are not deposited on the selected surfaces) are often volatile and being exhausted away.
Equally importance with the use of low resistance materials in interconnecting lines is the introduction of low dielectric constant materials (low-k dielectrics) for insulating between the interconnecting lines. The use of copper as a conductor and low-k dielectric as the dielectric material can provide lower delay time for advanced ICs. Low-k dielectrics are insulating dielectric materials that exhibit dielectric constants less than conventional IC dielectric materials such as silicon dioxide (k value of about 4), silicon nitride (k value of about 7), or silicon oxy-nitride (k value of about between 4 and 7).
Various low-k dielectrics have been introduced such as fluorine doped silicon dioxide (k value of about 3–3.6), carbon doped silicon dioxide (dielectric constant of about 2.5–3.3), fluorinated carbon (k value of about 2.5–3.6), and organic materials such as parylene (k value of about 3.8–3.6), polyimide (k value of about 3–3.7). Some of these materials have been successfully incorporated into the IC fabrication processes, but some materials have not because of various difficulties involved with the integration. The low-k dielectrics can be deposited by CVD or spin-on techniques.
Further research is focusing on porous low-k dielectrics because of the potential lower dielectric constant (2–3). The examples of porous low dielectric materials are porous hydrosilsesquioxane or porous methyl silsesquioxane, porous silica structures such as aerogel, low temperature deposited silicon carbon films, low temperature deposited Si—O—C films, methyl doped porous silica.
The porous low-k dielectrics present a significant integration problem such as low mechanical strength, poor dimensional stability, poor temperature stability, high moisture absorption, permeation, poor adhesion, large thermal expansion coefficient, and unstable stress level.
Currently, the most successful low-k dielectric films are polymer-based dielectrics. The polymer-based dielectrics can be inorganic polymers such as silicon-based polymers like SOG (spin-on glass). The polymer-based dielectrics can be organic polymers such as silicon-based polymers with higher organic contents, an aromatic hydrocarbon, poly (arylene ether) (PAE) films, benzocyclobutene (BCB) based films, polyimides or fluorinated polyimides, amorphous fluorinated carbon films, polytetrafluoroethylene (PTFE) films, or parylene.
Various companies have introduced a variety of organic polymer dielectric films. Schumacher offers VELOX, a non-fluorinated PAE material. Allied Signal's FLARE 2.0 product is also a PAE material. Dow Chemical's SiLK material is another aromatic polymer, an aromatic hydrocarbon containing no silicon or fluorine. Dow Chemical also offers Cyclotene BCB products as low dielectric material.
Among the problems associated with organic polymer low dielectric materials, the problem of plasma damage is very severe. Generally, plasma-excited etchants such as fluorine, chlorine, bromine, oxygen, or even nitrogen can etch many materials, though oxygen or nitrogen plasma have little effect on inorganic materials. However, plasma excited oxygen or nitrogen has a strong etching effect on organic polymer materials. Plasma oxygen is routinely used to strip photoresist, an organic polymer material.
The principle of CVD (chemical vapor deposition) or CVD related technique is the chemical reactions of appropriate precursors (special chemicals) in vapor phase. Taking the deposition of NLD titanium nitride (TiN) thin films as an example, the precursor, tetrakis(dimethylamino) titanium (TDMAT), vapor is introduced into the reactor, and then thermally decomposed into titanium nitride and other by-products on the surface of a substrate to form a raw TiN film which contaminated with carbon and hydrogen, the raw TiN film is further treated by nitrogen plasma to remove any contaminates. Therefore, CVD related deposition process techniques need thermal energy, or plasma energy, or both.
The processes that use the plasma energy are called plasma assisted processes (or plasma enhanced processes). The advantages of a plasma assisted process are a wider selection of precursors and the lower of substrate temperature can be used. The disadvantages of a plasma assisted process are the presence of highly energetic species due to plasma excitation which can cause damage of the substrates if not well-controlled. Plasma excited species can definitely damage organic polymer films. Such damages bring issues especially in the recent introduction of organic polymer low dielectric film as insulator for metal interconnects.
A typical plasma assisted process, called PECVD (plasma enhanced chemical vapor deposition), is a CVD process that employs plasma energy during the deposition period, and therefore cause the undesirable effect of etching the organic polymer underlayer before the formation of the deposited film. Another plasma assisted process, call PEALD (plasma enhanced atomic layer deposition), is an ALD process that employs plasma energy. In an ALD process, the precursors are sequentially introduced to the process chamber. The first precursor molecules are adsorbed on the substrate to form a saturated monolayer, and then the second precursor molecules are introduced and to react with the adsorbed first precursor molecules to form a monolayer of the target material. PEALD employs plasma energy during the introduction of the first precursors, or during the introduction of the second precursors, or during the introduction of both precursors. In all cases, the organic polymer underlayer is either totally or partially exposed to the plasma species and therefore causes the undesirable effect of etching the organic polymer underlayer before the formation of the deposited film.
Another plasma assisted process, called PENLD (plasma enhanced nanolayer deposition), is an NLD process that employs plasma energy. The NLD and PENLD processes are of the same authors and are disclosed in a co-pending application Ser. No. 09/954,244, filing date Sep. 10, 2001, inventors Tue Nguyen and Tai Dung Nguyen, now U.S. Pat. No. 6,756,318. The NLD process is a hybrid of the CVD and ALD processes. In NLD process, the precursors are introduced sequentially, similar to ALD process. But then the first precursors are deposited on the substrate to form a thin film with a thickness ranging from 0.5 to 5 nanometers, similar to CVD process. The second precursors are introduced after the removal of the first precursors to react with the deposited thin film. PENLD employs plasma energy in the deposition of the first precursors, or in the reaction of the second precursors or both. PENLD could cause damage to the organic polymer underlayer if the organic polymer underlayer is exposed to the plasma excited species during the process.